This invention relates generally to data communication and more particularly to a transmission line structure for bi-directional communication between four sources/receivers.
In many communication systems, such as digital data sent between integrated circuits, a driver send electrical waveforms to a receiver. To accomplish this, the signal may have to propagate through a series of transmission lines. To minimize reflections, these transmission lines are often constructed such that their characteristic impedance (Z0) is the same as the driver impedance, the receiver impedance, or both. For high-speed connections, it is desirable for the driver, receiver, and the transmission line to all have the same impedance. This helps produce a system where there are no reflections on the transmission line or its ends. For the simplest case of one driver connected to one receiver, matching the driver and receiver and transmission line is quite simple.
Unfortunately, where a driver sends a signal along a transmission line to several receivers (or integrated circuits), producing a system with no reflections becomes more difficult. These systems (or busses) are typically called multi-drop busses.
Multi-drop busses typically generate multiple reflections because of impedance mismatches at each transmission line branch or each receiver. These multiple reflections can combine in complex ways thereby making design of the whole system difficult and complex. Often, a design that has to deal with these multiple reflections will require segments of transmission lines with many different characteristic impedances. This further complicates the design and layout of the system.
A four-drop bus has each driver or receiver terminated at the characteristic impedance of Z0. Each driver or receiver is connected to a segment of transmission line with a characteristic impedance of Z0. Two of these segments are connected at a first point. The other two of these segments are connected at a second point. The first and second points are connected by a central transmission line with a characteristic impedance of Z0/2.